As discussed in Section 10.2.1.3, there has been exponential growth in patent activity surrounding aqueous PU dispersions (PUDs). A significant amount of this protection of intellectual assets has been the application of PUDs to coatings. Environmental and occupational safety and health regulations to reduce exposure to organic solvents has driven innovation and adoption of aqueous dispersion technology [46, 47]. Today PUD coatings are accepted products for coating wood, automobiles, textile fibers, leathers, fiber glass, floor polishes, and waxes among others.
Batch and continuous methods of producing PUDs were introduced in Section 10.2.1.3. PUDs can also be made in small-scale batches for the purpose of trialing coating formulations. Such a procedure might be done on the subliter scale in a laboratory hood. An example of such a preparation follows:
A three necked flask is placed in a heating mantle and the prepolymer components: polyol, dimethylol propionic acid (DMPA), diol chain extender in the solvent dipropylene glycol dimethyl ether is weighed in. The solvent is added at 1 part to 3 parts polyol. The mixture is stirred with an overhead stirrer under a steady stream of N2 and heated to a temperature of ca. 70 °C. An isocyanate such as H12MDI, IPDI or HDI is added dropwise to the stirring mixture using an addition funnel not allowing the reaction exotherm to increase the flask temperature more than ca. 5 °C. The hard segment components (isocyanate, diol, DMPA) are added in amounts to create a desired hard segment volume—usually 40-55% of the final PUD. After allowing the urethane reactions to proceed for 3-5 h at 70 °C, triethyl amine is added in 1:1
TABLE 10.10 Illustrative properties obtained for waterborne polyurethane coating obtained by a dispersion process such as specified in the section
stoichiometric ratio to DMPA and allowed to neutralize the acid for ca. 15min. The diluted polymer is poured into a plastic bottle and allowed to cool to ca. 50 °C after which the polymer solution is diluted with water (~1 part water to 1 part polymer) and stirred at high shear rate with a mixer such as a cowels blade. The polymer is allowed to complete by addition of a diamine such as ethylene diamine or 1,2 propylene diamine and allowed to agitate for approximately 1 day. The dispersion is then filtered through an appropriate mesh filter. This dispersion is now ready for coating substrates. A rheology modifier may be added to improve coating aesthetics. The coating can be allowed to dry passively for 7 days. Properties of a PUD coating such as described are given in Table 10.10.